Spray-drying process

ABSTRACT

The spray-drying of solutions of chlorinated polydienes is carried out by atomizing the solution into an evaporation zone maintained at a temperature below 75* C through which gas is flowing.

United States Patent Felstead [45] July 18, 1972 s41 SPRAY-DRYINGPROCESS 3,241,600 3/1966 Whitehouse 159/48 3,320,220 5/1967 Drusco etal... ....260/96 X [721 England 3,437,645 4/1969 Paige et al ....26o/96x [73] Assignee: Imperial Chemical Industries Limited, 2,345,507 3/1944Smith ..260/772 London, England 3,499,883 3/ 1970 Rosen ..260/94.7 HA3,501,449 3/1970 Thorpe et a1... ..260/94.7 HA [22] 3,442,884 5/1969HOCh et al .;.260/94.7 HA [2]] App]. No.: 2,668 3,547,865 12/1970 l-lochet a1 ..260/94.7 HA

Primary Examiner-Norman Yudkoff [301 Assistant Examiner-6.1. Sofer Jan.27, 1969 Great Britain ..4,445/69 Attorney-Cushman, Darby & Cushman [52]US. Cl. ..159/48, 159/4 CC, 260/94.7 HA [57] ABSTRACT [51] Int. Cl...B0ld 1/16, CO8! 1/88, F2619 3/12,

, Cosd 5/06 The spray-drying of solutions of chlorinated polydlenes 18car- 581 mm of Search ..159/48, 48 L; 260/92.3, 96, fled by ammiling the$011150" w 260/94 7 772, 9 HA, 9 7 maintained at a temperature below 75Cthrough which gas is flowing. [56] References Cited 10 Claim 1 m I mUNITED STATES PATENTS 2,426,127 8/1947 Thomas et al ..260/96 XPRESSURIZED SOLUTION OF CHLORINATED POLYMER IN SOLVENT, 5.6. HAVING ABOIL- INC POINT GREATER THAN 75C SOLVENT VAPOR, GAS AND DRIED S'OLIDPARTICLES OF POLYMER SPRAY NOZZ LE HEATED GAS as. AIR, AT A TEMPERATURET0 mm- TAI N TEMPERATURE OF CHAMBER PATENTED JUL 1 8 m2 SOLVENT VAPOR,GAS AND omen souo PARTICLES OF POLYMER $PRAY NOZZLE A SPRAY DRYINGCHAMBER, E. G. AT ATMOSPHERIC PRESSURE AND LESS THAN 75C -HEATED GASE.G. AIR, AT

A TEMPERATURE TO MAIN- TAIN TEMPERATURE OF CHAMBER SPRAY-DRYING PROCESSThis invention relates to a process for spray-drying polymer solutionsand in particular to a process for the spray-drying of solutions ofchlorinated diene polymers.

It is known that solutions and suspensions of some polymers may berapidly evaporated to form dry particles by spray-drying i.e.evaporation of solvent by spraying the solution into a hot atmosphere.

Many polymer solutions are not amenable to spray-drying either becausethey are difficult to atomize or because the solid particles are toofluffy, tacky or otherwise unsuitable to extract from the dryingapparatus.

Chlorinated diene polymers fall into the category of polymers difiicultto extract rapidly from solution by spraydrying and preliminary attemptswere not successful. We have now found that if the solution is atomizedcontinuously into an atmosphere at the correct temperature solidparticles are obtained which may be removed continuously from the dryingapparatus.

Thus according to our invention we provide a method of spray-drying asolution of a chlorine-containing polymer derived from an aliphaticdiene which comprises atomizing the solution into an evaporation zonewherein the said solution encounters a flowing gas, the temperaturewithin the said zone being less than 75 C.

Whereas the use of temperatures greater than 75 C produces powders whichare difiicult to handle, the present invention provides polymerparticles which are free-flowing and which have a packing densitygreater than 40 gm/liter, and may be in the range 150-350 gm/liter, asthey emerge from the dryer. in this form they may be removed readilyfrom the drying apparatus in an air stream and separated continuously bya cyclone separator. Other continuous methods of separation of the solidmay be employed, for example a form of deposition on to anelectrostatically-charged plate from which polymer is removed byscraping, or an electrostaticallycharged surface forming part of aconveyor belt.

When the polymer emerges from the spray-dryer some solvent may beentrained in the particles, and the powders may be given a furtherdrying if desired but after the present spraydrying process they areusually sufficiently dry to be ballmilled. Ball-milling is theconventional method used to produce particles of chlorinated dienepolymers having the desired packing density and may be carried outsubsequent to the spray-drying process to produce polymer of even higherpacking density, for example a packing density of 400-450 gm/liter isdesirable in the case of chlorinated polyisoprenes.

Polymers which are conveniently separated from solution by this processare chlorine-containing polymers derived from aliphatic dienes either bythe chlorination of a polymer (for example a synthetic polyisoprene, asynthetic polybutadiene or a natural rubber) or by the polymerization ofa chlorinecontaining monomer (for example 2-chloroisoprene).

The process is especially applicable to polymers obtained by thechlorination of natural and synthetic polyisoprenes in solution, forexample polymers having a chlorine content within the range 5075 percentby weight and preferably within the range 62-68 percent by weight.

The solution to be dried by the process of this invention is that of oneof the polymers hereinbefore described dissolved in a suitable solvent.The solvent will be most conveniently that in which the chlorination hasbeen carried out, and the most common solvent for this purpose is carbontetrachloride. Other saturated solvents containing a large proportion ofchlorine may be used, for example one or more of chloroform,dichlorobromomethane, bromotrichloromethane, tetrachloroethane andtetrachlorodifluoroethane, either alone or used in mixtures with carbontetrachloride. The process operates most efficiently with solventshaving boiling points close to that of carbon tetrachloride andpreferably the solution comprises a major proportion of carbontetrachloride.

It is not necessary that the solvents be free from water but ifsufficient water is present to form a second liquid phase it ispreferred to add an emulsifying agent in order to maintain a uniform andstable suspension of water throughout the Grade 20 c.p.; Alloprene is aRegistered Trade Mark) in carbon tetrachloride has been found to atomizesatisfactorily at concentrations below 5 percent by weight but it ispreferable to reduce the concentration to approximately 4 percent, atwhich concentration the viscosity of the solution was observed to be 3.6centipoises. If the concentration is reduced well below this level, forinstance to 1 percent the solution may be sprayed satisfactorily but theprocess is 'inefi'icient because, for a unit weight of solid polymerproduced, large amounts of heat energy are required to evaporate thesolvent. Therefore a concentration of solution close to theconcentration limit for suitable atomization should be chosen asexemplified above.

The polymer solution is conveyed to an atomizer for example by being fedunder pressure to a spray nozzle; the exact pressure is not criticalprovided it is sufficient to generate a mist-like spray. The spray ofpolymer solution is optionally arranged to emerge either from a singleorifice designed to produce a hollow conical-shaped discharge or from aplurality of such nozzles suitably arranged. A fine-mist spray should beproduced, for example a spray having solution droplets between 10 and500 microns in diameter, preferably between 20 and 100 microns.

The spray nozzle or noules are arranged to discharge the solution into achamber preferably at atmospheric pressure or lower in which a gas isflowing, for example in a stream or in circulatory currents. The flow ofgas through the evaporation, zone in the chamber may be obtained in avariety of ways for example by the injection of pressurized air or by amechanically-driven impeller situated either inside or outside thechamber. The movement of gas in the chamber is preferably designed tosweep the partially-dried particles away from the walls of the chamberto minimize adhesion thereto.

Heat is supplied to the evaporation zone preferably by heating the gasentering the chamber to a temperature considerably above the temperatureat which the evaporation zone is maintained. Some of the heat is lostfrom the incoming air by conduction to the walls or other parts of theapparatus and some is used to provide the latent heat of vaporization ofthe solvent. The evaporation zone is maintained at a substantiallyuniform temperature and this temperature may conveniently be measuredunder operating conditions near the vapor outlet of the chamber. Thetemperature of the said zone, measured as the temperature of thegaseous/solid suspension when steady-state conditions have beenestablished, is below 75 C; a convenient range is, for example, 20 to 65C but preferably the range is from 25 to 55 C in order to producepolymer particles of the desired packing density as hereinbeforedescribed.

The temperature of the gas entering the chamber will usually be above Cand may be between C and 200 C in order to maintain the requiredtemperature at the outlet. With suitable flexibility in the control ofheat input to the incoming gas and the rate of spraying of the polymersolution a convenient heat balance may be established such that aconstant temperature is observed at the outlet of the chamber and auniform weight of dried-polymer per unit time is collected.

The gas used to convey heat into the evaporation chamber and carry thesolid product from the chamber may be any gas not reactive to thepolymer at the temperature of the drying process. Air is the mostconvenient gas for this purpose and is preferably employed but othergases, forexample nitrogen, argon or carbon dioxide, would be suitablebut more expensive. Optionally solvent vapor and if desired other gasestoo may be re-circulated to the evaporation zone after the polymerparticles have been separated: this provides a method by which at anychosen temperature and pressure the rate of evaporation of the dropletsof solution may be further controlled.

The invention is illustrated but not limited by the following Examples:

EXAMPLE 1 A solution containing 4 percent by weight of chlorinatedpolyisoprene in carbon tetrachloride was fed at a pressure of 100 p.s.i.to a drying chamber into which hot air was flowing from an independentpipe at a rate of 55 cu.ft. per minute. The chlorinated rubber solutionwas atomized into a hollow conical spray through a nozzle having a sprayangle of 80 at a rate of 6 liters per hour. The solid particles ofpolymer produced were carried out of the chamber by the air flow andseparated in a conventional cyclone separator.

The packing density of the product obtained by conducting the aboveexperiment with air at different temperatures was measured. The resultsare summarized in Table 1.

TABLE 1 Temperature of Air C Packing Density Solvent of Product byweight lnletOutlet g/liter in product l6555 I20 I9.() [5055 1 l l9.516852 120 l9.l 19565 50 13.4 1 I445 200 l4.8 9636 2l4 l6.8

EXAMPLE 2 A spray-drying experiment was conducted with identicalconcentration of solution, feed pressure, and flow rates of air andpolymer solution as in Example l. in this experiment the differencebeing that the air used and a portion of the solvent vapor werecontinuously reheated and returned to the evaporation chamber after thepolymer particles had been separated in a cyclone separator. The resultsobtained are shown in Table 2.

TABLE -2 Temperature of Air "C Packing Density Solvent of Product byweight lnletOutlet g/liter in product lclaim:

l. A method of spray-drying a solution of a chlorine-containing polymerderived from an aliphatic diene which contains from 50 to 75 percent byweight of chlorine, the step of which consists essentially of atomizinga solution of said polymer in a solvent therefor which solvent isselected from the group consisting of carbon tetrachloride,dichlorobromomethane, bromotrichloromethane,

tetrachloroethane, tetrachlorodifluoroethane and mixtures of a majorportion of carbontetrachloride and a minor proportion of at least onemember selected from the group consisting of dichlorobromomethane,bromotrichloromethane, tetrachloroethane and tetrachlorodifluoroethaneinto .an

evaporation zone wherein the solution encounters a flowing I that inwhich the polymer was chlorinated.

4. A method as claimed in claim 1 wherein the solvent has a boilingpoint close to the boiling point of carbontetrachloride.

5. A method as claimed in claim 1 wherein the polymer is a chlorinatedpolyisoprene or a chlorinated natural rubber.

6. A method as claimed in claim 5 wherein the polymer contains from 62to 68 percent by weight of chlorine.

7. A method as claimed in claim 1 wherein the temperature within theevaporation zone is from 20 to 65 C.

8. A method as claimed in claim 7 wherein the temperature within theevaporation zone is from 25 to 55 C.

9. A method as claimed in claim 1 wherein the solution is atomizedthrough one or more spray-nozzles.

10. A method as claimed in claim 9 wherein the solution is atomizedthrough a spray-nozzle producing a hollow, conical discharge.

' I. h i i

2. A method as claimed in claim 1 wherein the concentration of polymerin solution is less than 5 percent by weight.
 3. A method as claimed inclaim 1 wherein the solvent is that in which the polymer waschlorinated.
 4. A method as claimed in claim 1 wherein the solvent has aboiling point close to the boiling point of carbontetrachloride.
 5. Amethod as claimed in claim 1 wherein the polymer is a chlorinatedpolyisoprene or a chlorinated natural rubber.
 6. A method as claimed inclaim 5 wherein the polymer contains from 62 to 68 percent by weight ofchlorine.
 7. A method as claimed in claim 1 wherein the temperaturewithin the evaporation zone is from 20* to 65* C.
 8. A method as claimedin claim 7 wherein the temperature within the evaporation zone is from25* to 55* C.
 9. A method as claimed in claim 1 wherein the solution isatomized through one or more spray-nozzles.
 10. A method as claimed inclaim 9 wherein the solution is atomized through a spray-nozzleproducing a hollow, conical discharge.